Ontogenesis of mericarps of bitter fennel (Foeniculum vulgare Mill. var. vulgare) as related to t-anethole accumulation

Bitter fennel (Foeniculum vulgare Mill. var. vulgare, Apiaceae) is a common perennial hemicryptophyte, used since antiquity as a medicinal and aromatic herb in the Mediterranean basin. We describe the anatomical specialization and some of the factors that affect phenylpropene accumulation in the fruits of a t-anethole-rich chemotype during development. Histological examination of fruits by longitudinal sections indicated that each oil duct is an elongated cavity with a series of internal septa at 200-400 μm intervals. Oleoresin accumulation is accompanied by an increase in the oil duct area, as observed in transversal cross sections in early stages of development (from the yellow bud stage to the open flower stage). Upon maturation, oleoresin is further accumulated due to increased duct volume as a result of duct elongation. The main component of the oleoresin, t-anethole, is synthesized during flowering and in the early stages of fruit development, as monitored by the levels of S-adenosine methionine: t-anol O-methyltransferase activity. Upon transition from the waxy fruit stage (28 days after the yellow bud stage) to fully ripe fruit (42 days after the yellow bud stage) O-methyltransferase activity apparently ceases, indicating the cessation of de novo biosynthesis, while oleoresin levels remain constant, likely due to a lack of further metabolism and minimal volatilization, as indicated by apparent high lignification of the cells lining the oil ducts.